Method and apparatus for applying a Mid-IR graded-index microstructure to an optical fiber tip to achieve anti-reflective properties

ABSTRACT

A method and apparatus for applying a mid-IR graded microstructure to the end of an As2S3 optical fiber are presented herein. The method and apparatus transfer a microstructure from a negative imprint on a nickel shim to an As2S3 fiber tip with minimal shape distortion and minimal damage-threshold impact resulting in large gains in anti-reflective properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of the filing date of U.S.Provisional Application No. 61/328,288 filed on Apr. 27, 2010, theentire contents of which are incorporated by reference hereto.

STATEMENT OF GOVERNMENT INTEREST

The claimed subject matter was reduced to practice with United StatesGovernment support under Contract No. N00173-05-C-6020 awarded by theUnited States Naval Research Laboratory. Accordingly, the United StatesGovernment has certain rights in the claimed subject matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed subject matter relates to a method and apparatus fortransferring a microstructure from a negative imprint on a nickel shimto an As2S3 optical fiber tip with minimal shape distortion and minimaldamage-threshold impacts, thereby improving the anti-reflectiveproperties of the As2S3 optical fiber.

However, the method and apparatus herein presented are not limited inapplication to As2S3 optical fibers. In fact, the apparatus and methodherein presented can be practiced with other optical fiber materialshaving melting points below 600° C.

2. Brief Description of Related Art

Optical fibers can be used in a great number of applications in theMid-IR wavelength region including sensing, imaging and processing.These optical fibers have large refractive indexes, ranging from about2.3 to about 2.9. Air, on the other hand, has a refractive index of 1.This large difference in refractive index between the optical fibers andair leads to signal losses at the optical fiber/air interface. In some,applications these signal losses at the optical fiber/air interface canamount to 25% or more.

In order to prevent signal losses, some have turned to applyinganti-reflective coatings to polished fiber tips. However, polishing canoftentimes lead to optical fiber fracture in delicate fiber materials.Furthermore, anti-reflective coatings often exhibit adhesion problemsand rapid degradation as a result of exposure to high intensity signalradiation.

SUMMARY OF THE INVENTION

Therefore, a need exists for an improved method and apparatus forapplying an anti-reflective treatment to mid-IR optical fibers that isreliable, efficient and does not damage the optical fibers.

In one embodiment, the disclosed subject matter relates to a method forpreventing reflection losses in optical fibers, the method comprisingthe steps of heating an optical fiber tip to form a heated optical fibertip, flattening the heated optical fiber tip to form a flattened opticalfiber tip, heating the flattened optical fiber tip and imprinting amicrostructure onto the flattened optical fiber tip.

In another embodiment, the disclosed subject matter relates to methodfor preventing reflection losses in properly terminated As2S3 fibers,the method comprising securing a properly terminated As2S3 fiber into aferrule, so that a tip of the properly terminated As2S3 fiber protrudesabout 1 mm to 2 mm from the ferrule, fastening the ferrule to a fixture,lowering the fixture onto a heating surface, such that the heatingsurface transfers heat to the properly terminated As2S3 fiber tipwithout touching the properly terminated As2S3 fiber tip, adjusting theorientation of the fixture with a hollow cylinder placed between thefixture and the heating surface to ensure perpendicularity of theproperly terminated As2S3 fiber tip relative to the heating surface,lowering the fixture so that the properly terminated As2S3 fiber tipcontacts the heating surface, replacing the heating surface with a hotimprinting surface, lowering the fixture onto the hot imprintingsurface, such that the hot imprinting surface transfers heat to theproperly terminated As2S3 fiber tip without touching the properlyterminated As2S3 fiber tip and lowering the fixture so that the properlyterminated As2S3 fiber tip contacts the hot imprinting surface.

In another embodiment the disclosed subject matter relates to anapparatus for creating an antireflective tip in an optical fiber, theapparatus comprising a fixture that is primarily capable oftranslational motion, with tip/tilt rotational fine-adjustment viagoniometer or similar device, a heating element positioned along atranslational axis of the fixture and a shaping member disposed on theheating element.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be better understood from the detailed descriptiongiven below and by reference to the attached drawings in which:

FIG. 1A is a flowchart depicting an embodiment of the method forpreventing reflection losses in optical fibers.

FIG. 1B is a flowchart depicting an embodiment of the method forpreventing reflection losses in optical fibers.

FIG. 2 is a schematical view an embodiment of the apparatus for creatingan antireflective tip in an optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

As used herein the term “properly terminated fiber,” or “appropriatelyterminated optical fiber,” or “properly terminated As2S3 fiber” means anoptical fiber, including an As2S3 fiber and any other optical fiberhaving a melting point below 600° C., that has been previously cleavedand glued into a Zirconia Ferrule, such that the optical fiber tipprotrudes out about one to two diameters.

Referring to FIG. 1A, a preferred embodiment of the claimed method isdepicted in flow chart 100. In step 103, using standard optic practices,a flat shaping member is cleaned, first using acetone and subsequentlyusing methanol. In step 106, ionized air is applied to the flat shapingmember in order to remove any excess acetone and/or methanol left overfrom step 103. In step 109, a flat shaping member is inspected forcleanliness under a high power, long focal length digital microscope,having at least 80× magnification capabilities.

In order to ensure proper cleanliness during this stage of the process,if microscopic inspection in step 109 reveals that further cleaning isnecessary, the cleaning procedure should be repeated beginning with step103, until microscopic inspection in step 109 reveals that the flatshaping member is clean.

In step 112, using standard optic cleaning practices, an imprintingmember is cleaned using methanol. In step 115, ionized air is applied tothe imprinting member in order to remove any excess methanol left overfrom step 112. The imprinting member is then cleaned using an ArgonPlasma Cleaner (such as “Plasma Preen” made by Terra Universal, Inc)(step 118), and inspected for cleanliness (step 121) under a high power,long focal length digital microscope, having at least 80× magnificationcapabilities.

In order to ensure proper cleanliness during this stage of the process,if microscopic inspection in step 121 reveals that further cleaning isnecessary, the cleaning procedure should be repeated beginning with step112, until microscopic inspection in step 121 reveals that the flatshaping member is clean.

In step 124, the flat shaping member is placed on a temperatureadjustable heating unit, such as a hot plate. While the shaping memberis heating up (in step 124), in step 127 a properly terminated As2S3fiber is inserted into a fastening fixture, usually made of steel. TheAs2S3 fiber ferrule is inserted into the fixture such that theZirconia-clad tip protrudes about 1 mm to about 2 mm from the end of thefastening fixture. The fastening fixture holds the Zirconia ferruleparallel to the primary axis of translation and features a largerperpendicular surface which can be used as a reference surface in thenext step (the hollow cylinder adjustment). The fixture is capable oftranslational motion, so that as the fixture moves down, the fixturemoves toward the shaping member positioned directly below the fixtureand as the fixture moves up, the fixture moves away from the shapingmember positioned directly below the fixture.

Once the ferrule and optic fiber are fastened in the fixture (step 127),in step 130, a hollow cylinder with flat, parallel ends is placed on topof the flat shaping member and the fixture is lowered so as to touch theopposite end of the cylinder. The tip/tilt adjustment feature is used(step 133) to make sure the fixture is flush with the cylinder end(adjusted by eye). This ensures that the ferrule/optic fiber assembly'stranslational axis is normal to the surface of the shaping member.Further, because the cylinder is hollow, only the outer surface of thefixture contacts the ring, while the optic fiber tip remains untouchedwithin the hollow space.

In step 133, goniometers are adjusted to ensure that the ferrule/opticfiber assembly is normal (along the axis of translational motion) to thetop surface of the shaping member. Thus once the ring is removed thebottom surface of the ferrule and the optic fiber tip contained withinit are nominally parallel to the flat shaping member (to within thetolerances of the assemblies).

Referring to FIG. 1B, in step 136, the temperature of the flat shapingmember is allowed to stabilize with the temperature of a hot platesurface, to a temperature range of about 170° C. to about 270° C. It isimportant to ensure the flattening member stabilizes within theabove-mentioned temperature range because a lower temperature may resultin defective flattening, while a higher temperature could result inoptic fiber damage.

Once the flattening member has stabilized at the desired temperature(step 136), in step 139 the fixture is lowered bringing theferrule/optic fiber assembly toward the heated flattening member. Sincethe optic fiber tip protrudes from the ferrule, as the fixture movestoward the heated flattening member, the optic fiber tip is closer tothe heated flattening member than any portion of the ferrule. Thefixture should move down toward the heated flat shaping member until theoptic fiber tip is about 100 μm to about 200 μm from the heated flatshaping member. Once the optic fiber tip is within this desired range,the fixture is held in place for about 60 seconds. This permits the tipto be heated radiatively and by air convection due to its closeproximity to the heated surface.

At the end of the 60 seconds of step 139, the fixture again moves downtoward the heated flat shaping member until the optic fiber tip contactsthe heated surface of the flat shaping member (step 142). To ensureappropriate flattening of the optic fiber tip, in step 142 a prescribedpressure of about 3,000 PSI to about 144,000 PSI is applied on the opticfiber tip against the flat surface of the heated flat shaping member.The period of time during which contact and pressure are applied canvary. In fact, contact and pressure can be maintained for a period ofabout 10 seconds to about 300 seconds (Note: time, temperature andpressure are co-dependent variables—reducing one quantity can often bemade up by increasing another).

In step 145, the pressure on the optic fiber tip is removed and thefixture is moved, away from the heated surface of the flat shapingmember. The optic fiber tip at the end of step 145 should be flat,consistent with the surface of the heated flat shaping member.

In step 148, the flat shaping member is removed from hot plate andreplaced with the imprinting member. The imprinting member is allowed tostabilize to a temperature range of about 170° C. to about 270° C. Thesurface of the imprinting member closest to the optic fiber tip containsa negative imprint of the microstructure that will later be contacttransferred to the optic fiber tip on step 154 below. The microstructureon the imprinting member can consists of any desired patternarrangement, but usually contains a plurality of protrusions andrecesses. In one embodiment, the microstructure used is manufactured byTelAztec LLC of Burlington Mass. 01803.

Once the imprinting member's temperature is stabilized (step 148), instep 151 the fixture is lowered bringing the ferrule/optic fiberassembly toward the heated imprinting member. The fixture should movedown toward the heated imprinting member until the optic fiber tip isabout 100 μm to about 200 μm from the heated imprinting member. Once theoptic fiber tip is within this desired range, the fixture is held inplace for about 60 seconds. This permits the tip to be heatedradiatively and by air convection due to its close proximity to theheated surface.

At the end of the 60 seconds of step 151, the fixture again moves towardthe heated imprinting member until the optic fiber tip contacts theheated imprinting member (step 154). To ensure appropriate imprinting ofthe optic fiber tip, a prescribed pressure of about 3,000 PSI to about80,000 PSI is be applied on the optic fiber tip against the surface ofthe imprinting member. Contact and pressure should be maintained for aperiod of about 30 seconds. The period of time during which contact andpressure are applied can vary. In fact, contact and pressure can bemaintained for a period of about 10 seconds to about 300 seconds (Note:time, temperature and pressure are co-dependent variables—reducing onequantity can often be made up by increasing another).

At the end of the 30 second period, in step 157 the pressure is removedand the fixture is moved away from the imprinting member. The opticfiber tip at the end of step 157 should have a microstructure,consistent with the surface of the imprinting member.

In step 160, the ferrule/optic fiber assembly is removed from thefixture and is inspected under appropriate magnification to ensureproper microstructure transfer.

Referring to FIG. 2, a preferred embodiment of the apparatus forcreating an antireflective tip in an optical fiber 200 (hereinafter “theapparatus”) is depicted. The apparatus 200 has a fixture 227 that iscapable of rotational motion. In one embodiment the fixture 227 iscapable of rotational motion along a y-axis that is perpendicular to awork bench 203. However, in other embodiments the fixture 227 may becapable of translational motion along the x-axis or z-axis.

The apparatus 200 also has a heating element 218 that is positionedalong the axis of translational motion of the fixture 227. The heatingelement 218 has a temperature sensing means 221 disposed on it. Thetemperature sensing means 221 can consist of a thermistor, a thermometeror any other temperature sensing device commercially available.

The axis of translational motion of the fixture 227 can vary. Forinstance, in one embodiment, where the axis of translational motioncorresponds to an axis of gravitational acceleration, in this embodimentthe heating element 218 can be positioned below the fixture 227, whichis disposed on a work bench 203. In an alternate embodiment, the heatingelement 218 can be position above the fixture 227. Further, in otherembodiments the axis of translational motion of the fixture 227 can beperpendicular to the axis of gravitational acceleration, in thisembodiment the heating element 218 can be positioned at any number ofpositions along the axis of translational motion of the fixture 227. Theheating element 218 can be any commercially available hot plate havingeither variable temperature of variable power settings.

Positioned on the heating element 218, there is a shaping member 224.The shaping member 224 is positioned so that at least a portion of theshaping member 224 intersects the axis of translational motion of thefixture 227. The shaping member 224 can have different patterns. Forinstance, in one embodiment the shaping member 224 has a substantiallyflat shape. In an alternate embodiment, the shaping member 224 can havea negative imprinting region having an irregular shape consisting of aplurality of protrusions and recessions.

Further, the shaping member 224 can have either single shaping patternor a multiple shaping patterns. For instance, in one embodiment theshaping member 224 can have a single substantially flat shaping pattern.In a different embodiment, the shaping member 224 can have a singlenegative imprinting region consisting of an irregular shape patternhaving a plurality of protrusions and recessions. In other embodiments,the shaping member 224 can have combinations of patterns located ondifferent regions of the shaping member 224. For instance in oneembodiment the shaping member 224 can have a first region that issubstantially flat and a second region that is substantially irregular.

Fastened to the fixture 227, there is a ferrule 233. The ferrule 233 canbe any type of commercially available and appropriate ferrule, includingsteel ferrules as long as the process of inserting the fiber into theferrule 233 does not damage it. The bare optical fiber 245 is glued intothe ferrule 233 in advance. The bare optical fiber 245 and ferrule 233assembly can have additional secondary ferrules, fixture adapters orarmor surrounding the ferrule-terminated fiber such that the assemblyfits snugly into the fixture 227. In a preferred embodiment, the bareoptical fiber 245 is an As2S3 fiber. Further, the bare optical fiber 245is terminated with a Zirconia ferrule nested inside a steel adapterplaced in the ferrule 233, so that at least of a portion of the bareoptical fiber 245 (a fiber tip) protrudes through the ferrule 233assembly.

Upon fastening to the fixture 227 the ferrule 233 and bare optical fiber245 moves along the translational axis of motion of the fixture 227,either toward or away from the shaping member 224. Guiding thetranslational motion there can be a rail 230 that is operativelyconnected to the fixture 227, so that the fixture 227 can move along therail 230. A connecting member 236 can be used to connect the rail 230 toz-axis goniometer 239 and x-axis goniometer 242, which allow fortwo-dimensional rotational adjustments of the fixture 227 along thez-axis and x-axis, respectively. In general, the goniometers' 239 242axes are set up perpendicular to one-another as well as perpendicular tothe primary axis of translation (y-axis in this example).

In one embodiment, there can be platform 251 connected to the x-axisgoniometer 242 and also operatively connected via a connecting member236 to translational stage 254, thereby providing translational motionto the fixture 227. In an alternate embodiment, the platform 251 can beconnected to the z-axis goniometer 239 and also be operatively connectedvia a connecting member 236 to translational stage 254. Translationalstage 254 provides translational motion to the fixture 227 along anydesired axis depending on the desired set up. For instance in oneembodiment, translational stage 254 may provide translational motionalong an axis normal (i.e. along a y-axis) to a work bench 203. In otherembodiments, the translational stage 254 may provide translationalmotion along an axis parallel (i.e. along an x-axis) to a work bench203.

Additional translational members may be provided in order to facilitatetwo or three dimensional adjustment capabilities. For instance, in oneembodiment a z-axis translational stage 257 can be operatively connectedto translational stage 254 to provide overall control of both y-axis andz-axis motion, respectively. Further, an x-axis translational stage 260can be operatively connected to z-axis translational stage 254 toprovide overall control of y-axis, z-axis and x-axis, respectively.

The apparatus 200 also has a pressure exerting member 248. The pressureexerting member 248 exerts pressure along the translational axis ofrotation of the fixture 227, and more particularly along the length ofthe bare optical fiber 245. In one embodiment the pressure exertingmember 248 can be a weight. In this embodiment, the weight can bedisposed on platform 251, where it transfers pressure to the fixture 227by gravity. In other embodiments, the pressure exerting member 248 canbe a hydraulic device, mechanical screw, or other pressure exertingdevice operatively connected to the fixture 227. The pressure exertingmember 248 is capable of exerting a pressure in the range of about 3,000PSI to about 144,000 PSI. In the case where weights are used, theapplied pressure is inferred by dividing the weight by the bare opticalfiber 245 surface area. In other embodiments, a scale or force-feedbacksensor can be used to measure the applied force for pressurecalculations.

A microscope 209 can be positioned within line of sight of the fixture227 in order to facilitate visual inspection of the bare optical fiber245 while the apparatus 200 is in use. In one embodiment, the microscope209 has a long focal length and has at least 80× magnificationcapabilities along with electronic video capabilities. The microscopecan be positioned on a microscope stand 206 and can be optionallyconnected via a video-signal cable 212 to a monitor 215 that displaysthe images captured by the microscope 209, thereby facilitationinspection of the bare optical fiber 245.

It is to be understood, that the above-described arrangements areintended solely to illustrate the application of the principles of thedisclosed subject matter. Numerous modifications and alternativearrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the disclosed subject matter inthe present Application. Accordingly, the appended claims are intendedto cover such modifications and alternative arrangements. Thus, whilethe disclosed subject matter of the present Application has beendescribed above with particularity and detail in connection with what ispresently deemed to be the most practical and preferred embodiments, itwill be apparent to those skilled in the art that numerousmodifications, including, but not limited to, variations in size,materials, shape, form, function and manner of operation, assembly anduse may be made without departing from the principles and concepts setforth herein.

What is claimed is:
 1. A method for preventing reflection losses inoptical fibers, the method comprising: heating an appropriatelyterminated As2S3 optical fiber tip to form a heated optical fiber tip;flattening the heated optical fiber tip to form a flattened opticalfiber tip; heating the flattened optical fiber tip; and imprinting amicrostructure onto the flattened optical fiber tip.
 2. The method ofclaim 1, further comprising applying pressure to the heated As2S3 fibertip against a substantially flat surface.
 3. The method of claim 1,further comprising applying pressure to the flattened As2S3 fiber tipagainst a negative imprinting surface.
 4. The method of claim 2, furthercomprising causing the pressure applied to the heated As2S3 fiber tip tobe in the range of about 3,000 PSI to about 80,000 PSI.
 5. The method ofclaim 3, further comprising causing the pressure applied to theflattened As2S3 fiber tip to be in the range of about 3,000 PSI to about80,000 PSI.
 6. The method of claim 2, further comprising causing theheated As2S3 fiber tip to be heated to a range of about 170° C. to about270° C.
 7. The method of claim 2, further comprising causing theflattened As2S3 fiber tip to be heated to a range of about 170° C. toabout 270° C.
 8. The method of claim 3, further comprising causing theimprinting surface to have a plurality of protrusions and recesses.
 9. Amethod for preventing reflection losses in properly terminated As2S3fibers, the method comprising: securing a properly terminated As2S3fiber into a ferrule, so that a tip of the properly terminated As2S3fiber protrudes about 1 mm to 2 mm from the ferrule; fastening theferrule to a fixture; lowering the fixture onto a heating surface, suchthat the heating surface transfers heat to the properly terminated As2S3fiber tip without touching the properly terminated As2S3 fiber tip;adjusting the orientation of the fixture with a hollow cylinder placedbetween the fixture and the heating surface to ensure perpendicularityof the properly terminated As2S3 fiber tip relative to the heatingsurface; lowering the fixture so that the properly terminated As2S3fiber tip contacts the heating surface; replacing the heating surfacewith a hot imprinting surface; lowering the fixture onto the hotimprinting surface, such that the hot imprinting surface transfers heatto the properly terminated As2S3 fiber tip without touching the properlyterminated As2S3 fiber tip; and lowering the fixture so that theproperly terminated As2S3 fiber tip contacts the hot imprinting surface.10. The method of claim 9, further comprising applying a pressure ofabout 3,000 PSI to about 144,000 PSI to the properly terminated As2S3fiber tip against the heating surface.
 11. The method of claim 10,further comprising applying a pressure of about 3,000 PSI to about144,000 PSI to the properly terminated As2S3 fiber tip against the hotimprinting surface.
 12. The method of claim 11, further comprisingremoving the properly terminated As2S3 fiber tip from heating surfaceafter a contact period of about 10 seconds to about 300 seconds.
 13. Themethod of claim 11, further comprising removing properly terminatedAs2S3 fiber tip from the hot imprinting surface after a contact periodof about 10 seconds to about 300 seconds.
 14. The method of claim 11,wherein the step of lowering the fixture onto a heating surface includeslowering the properly terminated As2S3 fiber tip to within about 100 μmto about 200 μm of the heating surface.
 15. The method of claim 11,wherein the step of lowering the fixture onto the hot imprinting surfaceincludes lowering the properly terminated As2S3 fiber tip to withinabout 100 μm to about 200 μm of the hot imprinting surface.